The present invention relates to a chemical mechanical polishing slurry. In particular, it relates to a chemical mechanical polishing slurry suitable as a polishing liquid used during forming a damascene metal interconnect in manufacturing a semiconductor device.
With regard to forming a semiconductor integrated circuit such as ULSI which has been significantly refined and compacted recently, copper has been expected to be a useful material for electric connection because of its good electromigration resistance and lower electrical resistance.
To date a copper interconnect is as follows, due to problems such as difficulty in patterning by dry etching. Specifically, a concave such as a trench and a connection hole is formed in an insulating film, a barrier metal film is formed on the surface, a copper film is deposited by plating such that the concave is filled with the material, and then the surface is polished to be flat by chemical mechanical polishing (hereinafter, referred to as xe2x80x9cCMPxe2x80x9d) until the surface of the insulating film except the concave area is completely exposed, to form electric connections such as a damascene interconnect in which the concave is filled with copper, a via plug and a contact plug.
There will be described a process for forming a damascene copper interconnect with reference to FIG. 1.
On a silicon substrate on which a semiconductor device has been formed (not shown) is formed a lower interconnect layer 1 consisting of an insulating film comprising a lower interconnect (not shown). Then, as shown in FIG. 1(a), on the silicon substrate are sequentially formed a silicon nitride film 2 and a silicon oxide film 3. On the silicon oxide film 3 is formed a concave having an interconnect pattern and reaching the silicon nitride film 2.
Then, as shown in FIG. 1(b), a barrier metal film 4 is formed by sputtering. On the film is formed a copper film 5 over all the surface by plating such that the concave is filled with the material.
As shown in FIG. 1(c), the copper film 5 is polished by CMP to make the substrate surface flat. Polishing by CMP is continued until the metal over the silicon oxide film 3 is completely removed, as shown in FIG. 1(d).
In the above process for forming a damascene metal interconnect, a barrier metal film made of, for example, a tantalum metal such as Ta and TaN is formed as a base film for, e.g., preventing diffusion of a conductive metal such as copper into the insulating film. However, when simultaneously polishing such different materials, a polishing rate for the barrier metal film is significantly smaller than that for the conductive metal (e.g., copper) film. Specifically, when forming a damascene metal interconnect by CMP using a conventional polishing slurry, there is a significant difference between the polishing rates for the conductive metal film and the barrier metal film, which may cause dishing and erosion.
Dishing is a phenomenon that the conductive metal (e.g., copper) film in the concave is excessively polished so that the center of the metal film in the concave is depressed in relation to the plane of the insulating film on the substrate, as shown in FIG. 2. An adequately much polishing time is required for completely removing the barrier metal film 4 on the insulating film (silicon oxide film 3) because of a lower polishing rate for the barrier metal film. The polishing rate for the copper film is, however, higher than that for the barrier metal film, so that the copper film is excessively polished, resulting in dishing.
Erosion is a phenomenon that polishing in a dense interconnect area excessively proceeds in relation to that in a sparse area such as an isolated interconnect area so that the surface of the dense interconnect area becomes depressed in relation to the other surfaces, as shown in FIG. 1(d). When the dense interconnect area comprising many damascenes in the copper film 5 is considerably separated from the isolated interconnect area comprising less damascenes in the copper film 5 by, for example, an area without interconnects within the wafer, and the copper film 5 is polished faster than the barrier metal film 4 or the silicon oxide film 3 (insulating film), then a polishing pad pressure to the barrier metal film 4 or the silicon oxide film 3 in the dense interconnect area becomes higher than that in the isolated interconnect area. As a result, in the CMP process after exposing the barrier metal film 4 (the process of FIG. 1(c) and thereafter), there generates a difference in a polishing rate by CMP between the dense interconnect area and the isolated interconnect area, so that the insulating film in the dense interconnect area is excessively polished, resulting in erosion.
Dishing in the process for forming an electric connection in a semiconductor device as described above, may cause increase in an interconnection resistance and a connection resistance, and tends to cause electromigration, leading to poor reliability in the device. Erosion may adversely affect flatness in the substrate surface, which becomes more prominent in a multilayer structure, causing problems such as increase and dispersion in an interconnect resistance.
JP-A 8-83780 has described that dishing in a CMP process may be prevented by using a polishing slurry containing benzotriazole or its derivative and forming a protective film on a copper surface. JP-A 11-238709 has also described that a triazole derivative is contained in a CMP slurry for improving flatness in polishing copper. The technique, however, controls dishing by reducing a polishing rate for a copper film so that polishing of the copper film takes a longer time, leading to a lower throughput.
On the other hand, there has been disclosed a technique attempting to improve polishing property of a slurry by adjusting a viscosity of the polishing slurry.
JP-A 2001-169831 has described the use of a slurry composition comprising a non-reactive polyol as a thickener selected from the group consisting of glycerin and polyethylene glycol in order to prevent dishing in CMP. The slurry composition contains the thickener to 0.1 to 50 vol % to adjust a slurry viscosity to 3.4 to 12 cps (mPaxc2x7s).
JP-A 11-307484 has described the use of a polishing liquid exhibiting Bingham fluidity which comprises a carboxyvinyl polymer in order to prevent dishing in CMP. For a viscosity of the polishing liquid, a specific range has not been described, but a polishing liquid containing the carboxyvinyl polymer to 0.1 wt % has been exemplified.
JP-A 2000-160137 has described a polishing liquid for CMP, but not for metal polishing, comprising cerium oxide particles, water and an anionic surfactant which can be applied to shallow trench separation. The polishing liquid is prepared such that a polishing rate ratio is large between a silicon oxide film and a silicon nitride film. There has been described that a viscosity of the polishing liquid is preferably 1.0 to 2.5 mPaxc2x7s.
It has been, however, difficult to conduct CMP at a higher polishing rate while preventing dishing, only by adjusting a viscosity of a polishing slurry as is in the prior art.
An objective of this invention is to provide a chemical mechanical polishing slurry whereby a reliable damascene electric connection with excellent electric properties can be formed at a higher polishing rate, i.e., with a higher throughput, while preventing dishing, in polishing a metal film formed on an insulating film with a concave on a substrate.
This invention provides a chemical mechanical polishing slurry for polishing a metal film formed on an insulating film with a concave on a substrate wherein the slurry contains a thickener without an ionic group with an opposite sign to a charge on a polishing material surface to 0.001 wt % or more and less than 0.05 wt % to the total amount of the slurry and has a slurry viscosity of 1 mPaxc2x7s to 5 mPaxc2x7s both inclusive.
A chemical mechanical polishing slurry of this invention (hereinafter, referred to as a xe2x80x9cpolishing slurryxe2x80x9d) may be suitably used in polishing a metal film formed on an insulating film with a concave on a substrate by CMP.
The slurry may be effectively used in a process for forming a barrier metal film as a base film on an insulating film with a concave; forming a conductive metal film on the barrier metal film such that the concave is filled with the conductive metal film; and polishing by CMP the substrate to flatten the substrate surface until the barrier metal film or the insulating film is exposed so that an electric connection such as a damascene interconnect where the concave is filled with a conductive metal, a plug and a contact is formed. The slurry is more effective when the conductive metal is a copper-based metal and the barrier metal is a tantalum-based metal.
A polishing slurry of this invention may be used in CMP to form a reliable damascene electric connection with excellent electric properties at a higher polishing rate, i.e., a higher throughput while preventing dishing or erosion.